1. Field of the Invention
The present invention relates to a piezoelectric ink-jet printhead. More particularly, the present invention relates to a piezoelectric actuator that provides a driving force for ejecting ink in a piezoelectric ink-jet printhead, and a method for forming the piezoelectric actuator.
2. Description of the Related Art
Typically, ink-jet printheads are devices for printing a predetermined color image by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. In ink-jet printheads, ink ejection mechanisms may be generally categorized into two types. A first type is a thermally driven type, in which a heat source is employed to generate bubbles in ink to cause ink droplets to be ejected by an expansion force of the generated bubbles. A second type is a piezoelectrically driven type, in which ink is ejected by a pressure applied to the ink due to a deformation of a piezoelectric element.
FIG. 1A illustrates a schematic diagram of a conventional ink-jet printhead. Referring to FIG. 1A, a reservoir 2, a restrictor 3, a pressure chamber 4 and a nozzle 5 are formed in a passage forming plate 10. The reservoir 2, the restrictor 3, the pressure chamber 4 and the nozzle 5 are in flow communication and form an ink passage. A piezoelectric actuator 20 is provided on the passage forming plate 10. The reservoir 2 stores ink supplied from an ink container (not shown). The restrictor 3 is a passage through which the ink passes from the reservoir 2 to the pressure chamber 4. The pressure chamber 4 is filled with ink to be ejected and varies in volume as the piezoelectric actuator 20 is driven, thereby causing a change in pressure for ejecting or drawing in the ink. To this end, a portion of the passage forming plate 10 that forms an upper wall of the pressure chamber 4 serves as a vibration plate 1, which is deformed by the piezoelectric actuator 20.
The above-described conventional piezoelectric ink-jet printhead operates as follows. When the vibration plate 1 deforms due to a driving of the piezoelectric actuator 20, the volume of the pressure chamber 4 decreases, causing a change in a pressure within the pressure chamber 4, so that ink in the pressure chamber 4 is ejected through the nozzle 5. Then, when the vibration plate 1 is restored to an original shape thereof as the piezoelectric actuator 20 is driven, the volume of the pressure chamber 4 increases, causing a change in the volume, so that the ink stored in the reservoir 2 is drawn into pressure chamber 4 through the restrictor 3.
FIG. 1B illustrates a cross-sectional view of a conventional piezoelectric actuator taken along line A-A′ of FIG. 1A. Referring to FIG. 1B, the passage forming plate 10 is formed by fabricating a plurality of thin plates 11, 12, and 13, which may be formed of ceramics, metals, synthetic resin or silicon substrates or plastics, forming a portion of the ink passage, and then stacking and adhering the plurality of thin plates 11, 12, and 13 using an adhesive. As such, an upper plate 13, which is stacked above the pressure chamber 4, serves as the vibration plate 1. The piezoelectric actuator 20 includes a lower electrode 21, a piezoelectric film 22 and an upper electrode 23, which are sequentially stacked on the vibration plate 1. The lower electrode 21 is formed by sputtering a predetermined metal material on the vibration plate 1. The piezoelectric film 22 is formed by applying a paste-state ceramic material on the lower electrode 21 to a predetermined thickness by screen-printing and sintering the same. The upper electrode 23 is formed by applying a conductive material onto a surface of the piezoelectric film 22 by screen-printing or depositing by means of a sputterer, an evaporator or an E-beam irradiator.
However, since the piezoelectric film 22 formed by the conventional screen-printing method spreads laterally in view of paste-state material characteristics, it is more difficult to obtain a correctly sized, rectangular-shaped film. That is, the formed piezoelectric film 22 is thick at a middle portion thereof and thin at edge portions thereof. In an effort to remedy such a problem, conventionally, the upper electrode 23 is formed only on the thick, middle portion, and not on the thin, peripheral portions of the piezoelectric film 22. Even slight misplacement of the upper electrode 23 on the piezoelectric film 22 may result in a short between the upper electrode 23 and the lower electrode 21. After the piezoelectric film 22 is sintered, an electric field is applied to the piezoelectric film 22 to produce piezoelectric characteristics, which is called a polling process. In the polling process, a high electric field of about 10 kV/cm is applied between the upper electrode 23 and the lower electrode 21. If the upper electrode 23 is formed at the thin, peripheral portions of the piezoelectric film 22, a gap between the upper electrode 23 and the lower electrode 21 is reduced so that breakdown occurs, which causes cracking of the piezoelectric film 22 or adversely affects piezoelectric characteristics of the piezoelectric film 22.
To address these problems, the upper electrode 23 is formed only on the thick, middle portion of the piezoelectric film 22, rather than on the thin, peripheral portions. Thus, a width of the upper electrode 23 becomes much smaller than that of the piezoelectric film 22. As a result, the piezoelectric film 22 cannot sufficiently produce piezoelectric effects.
To obtain improved print quality, i.e., high resolution and fast printing, it becomes necessary to increase a nozzle density. To this end, a size of the pressure chamber 4 and a distance between adjacent pressure chambers 4 should be reduced, and a width of the piezoelectric film 22 should be reduced accordingly. If the width of the piezoelectric film 22 is reduced, however, it is more difficult to form the upper electrode 23 on the piezoelectric film 22 by the conventional piezoelectric actuator formation method, which impedes formation of the piezoelectric actuator 20. Conventionally, the margin of a line width is generally about 50 μm.